The present invention relates to data communications and more specifically to integrated circuits which receive data signals.
High-speed serial communications systems such as High-Speed SerDes (serializer-deserializer) (HSS) cores have a need to measure and adjust for characteristics of data signals they are designed to receive. Signal characteristics such as data transmission rate, voltage levels, duty cycle and jitter can vary as presented to a receiver. The amount of jitter present in a communications link between receiver and transmitter can affect the rate at which data can be transmitted on such link. Jitter can vary from one communication link to another, depending upon the amount of noise present on the communication link, the length of a cable which spans the link and other factors which can vary between respective communication links. The eyewidth of a data signal is a measure of the jitter on a communication link. When eyewidth is narrowed by jitter, reception of the data signal becomes more prone to error. When the eyewidth is narrowed beyond a certain size, corrective action such as lowering the data transmission rate may need to be taken to avoid the error rate from becoming excessive. In view of the foregoing, there is a need for a system and method for measuring the eyewidth of a data signal.
Methods of receiving data signals can vary. In intra-system communication systems, such as the internal computer bus system described in U.S. Pat. No. 7,225,370 to Chow et al. (“Chow et al.”), a clock signal is usually transmitted on a separate line from the data signal. The clock signal, being fed to both the transmitter and the receiver, maintains the operation of the transmitter and receiver in constant synchronization. Chow et al. describes a system for measuring the eyewidth of a data signal in such communication system synchronized by a separate clock line.
In contrast to the intra-system communication system described in Chow et al., in many inter-system communication systems, i.e., systems for communication between one system and another, a separate clock signal line is not available to maintain synchronization. A need exists to determine the eyewidth of a data signal in such system.